1. Field of the Invention
This invention relates to pneumatic tires in which island portions of the tread have a plurality of sipes. More particularly, the present invention relates to a pneumatic tire having at a given position of its island portion a sipe of a given shape.
2. Description of the Related Art
In the manufacture of tires having sipes in their tread island portion, a sector mold consisting of plural arc-shaped segments, each of which segments being provided at its inner surface with a plurality of sipeforming blades having a desired shape and size, is usually used during the vulcanization of a green tire, in which each blade is put into the green tire based on the inward displacement of each arc-shaped segment in the radial direction to form sipes in the tire.
In the conventional technique, a sector mold consisting of eight arc-shaped segments 31-38 as shown in FIG. 6 is used in the vulcanization of the green tire. In this case, each of these segments 31-38 is moved inward in the radial direction as shown by an arrow, whereby blades 39 disposed in the inner peripheral surface of each of the segments 31-38 and equally protruding inward in the radial direction are put into the green tire. Among these blades, as the blade approaches each end side of the segment (31-38) in the peripheral direction, an angle .alpha. between the protruding direction of the blade and the moving direction of the segment becomes large and hence a relative displacement of such an end-side blade 39 with respect to the green tire in the circumferential direction of the tire becomes large, so that a large external force in the circumferential direction of the tire is applied to the end-side blade 39 during the placement of the end-side blade into the green tire. As a result, the end-side blade 39 and hence the sipe formed thereby are deformed and the end-side blade 39 is damaged. This is a problem.
In order to solve the above problem, there have been proposed a method of forming sipes at a position remote from a position of the tread surface corresponding to an edge of the arc-shaped segment (31-38), a method of removing a sipe in the vicinity of the position of the tread surface corresponding to the edge of the arcshaped segment, and the like.
In these conventional techniques, however, a sipe does not exist in the vicinity of the position of the tread surface corresponding to the edge of the arc-shaped segment, so that the tread pattern changes at this position or general area to cause a problem of changing the performances of the tire. Moreover, according to the former technique that the sipe is formed at a position remote from the position of the tread surface corresponding to the edge of the segment, when the island portion of the tread is block, the sipes are particularly formed in the block at a one-side state in the circumferential direction of the tire, so that the rigidity of the block becomes ununiform to inversely cause the occurrence of uneven wear, and also if the length of the block in the circumferential direction of the tire is short, there is a problem that the block is broken off due to the unbalance of the sipes.